Currently, image analysis methods using hyperspectral images are being gradually proposed all over the world in the field of remote sensing technology using electro-optical images. Although approaches to such methods are being actively attempted in the fields of defense surveillance, geographic information and environmental surveillance, limited research is being carried out in the field of marine search platform (aircraft, ship) based image analysis.
FIG. 1 is an exemplary configuration diagram of an image system for capturing images of an accident scene in 244 bands according to the wavelength of light, captured via an airborne hyperspectral image.
FIG. 2 is a graph illustrating spectral reflectance characteristics according to a wavelength of light (spectral axis) of an arbitrary pixel selected by coordinates of a spatial axis and a time axis in the hyperspectral image shown in FIG. 1.
As shown in FIG. 1, a hyperspectral image is generally an image photographed in tens to hundreds of bands according to the wavelength of light. The hyperspectral image has high dimensional data composed of continuous bands having a wavelength width of about 10 nm in the wavelength range of 400 to 2500 nm.
Commonly used conventional image sensors capture multi-spectral images with only 3 to 10 bands having a large wavelength range of 100 to 200 nm. As shown in FIG. 2, since almost complete spectral characteristic curves can be obtained per pixel, it is possible to utilize spectral reflectance characteristics to detect distribution of specific materials, a target, a camouflaged body or the like, that are difficult to detect with existing multi-spectral images.
In the event of a marine accident, it is important to know the location of the disabled ship and the persons overboard accurately and quickly for rapid rescue and prevention of additional accidents.
In particular, since a marine accident requires a marine search to be carried out over a wide area including the range of drift movement, the development of a real-time analysis system based on an airborne image that allows quick observation of a wide sea area.
However, in the case of conventional technology based on general cameras or multi-spectral observation images, the reflectivity or morphological feature differences between a target material (ship body, overboard persons) and a background material (seawater) is used for target detection. Therefore, if the characteristics of the reflectivity within the observed band range are similar due to the color of the target material or the state of the sea, or if the size of the detected object is smaller than or similar to the spatial resolution of the image pixel which causes difficulties in morphological feature analysis, a problem arises wherein the target on sea is excluded from the detection.
In general, when a damaged or sinking ship or an overboard person due to a marine accident is detected, the pixel observation value of the subject to be detected is shown in a form wherein the spectral reflectivity of target materials as well as background materials (seawater) or moving objects (lifeboat, vest, etc.) are mixed. Therefore, when the conventional technology for detecting targets at sea based on multi-spectral images, the rate of missed detection is very high.
As a result, these problems can grow due to the decrease of spatial resolution and spectral sensitivity per pixel at the time of an aerial wide area observation. Any misses in detection in a marine accident search can cause fatal casualties.
Therefore, it is necessary to develop a detection system based on airborne hyperspectral images with a high detection rate and stability by utilizing spectral characteristics per pixel for quick and accurate detection and initial action.
The airborne observation hyperspectral sensor captures a surface area of a predetermined area corresponding to a predetermined spatial resolution per pixel.
Such observed surface area is represented by a quantitative sum of the reflected energies of a homogeneous single material or a mixture of two or more materials. The occupation ratio of each of the constituent materials within mixed pixels can be analyzed via a spectral mixture analysis.
In order to perform such a spectral mixture analysis, unique spectral reflectance characteristic information of each material is required. For this purpose, a single material pixel component is extracted from an image data to directly form an endmember and the spectral reflectance characteristic of the same is used, or a spectral library constructed via lab or field measurements is used.
There is an advantage in that when extracting directly from an image, it is possible to perform a spectral mixture analysis directly without prior information. However, if there is no pixel covered only with a single material, it is impossible to perform an accurate mixture analysis. There is a limit in that additional operations must be performed to confirm the material of the extracted end member.
On the other hand, when a spectral library is used, it is necessary to preliminarily construct a spectral library of the target materials, and it is necessary to take into consideration the difference in absolute values of spectral signals depending on weather and light conditions. However, it is possible to detect a target in an accurate and stable manner.
Therefore, the present inventors have found a method for detecting disabled ships and persons overboard based on airborne hyperspectral images, which is capable of finding characteristics for each subject to be searched, by considering the spectral characteristics of the subject to be searched when the difference between the subject to be searched and the surrounding environment is small and difficult to identify.